Charging Li-ion batteries in parallel
The essential bit [not explicitly mentioned in the other answers] is [by all accounts] matching the internal resistances of the cells. Which according to sellers can only be done reliably at factory. 

The MIT boffins have studied this too and quantified the impact of mismatched resistance: 


  a 20% difference in cell internal resistance between two cells cycled in parallel can lead to approximately 40% reduction in cycle life when compared to two cells parallel-connected with very similar internal resistance


And since you'll probably be interested in measuring this internal resistance yourself, this is how they've done it for 2.2 Ah batteries:


  Internal resistance
  was measured at 50% state of charge (SOC) with a 15 s DC pulse of
  40 A (17C). While there is no commonly accepted standard for
  measuring the internal resistance of lithium-ion batteries, we chose
  this current and time profile because it is relevant to the duty cycle
  seen by these cells in hybrid vehicles and power tools. A comparison
  of several methods for the internal resistance of lithium-ion cells is
  provided by Schweiger et al. [3]. The 15 s current pulse allows the
  effects of the mass-transfer limited reaction to show. Longer delay
  times can lead to significant self-heating of the cell which affects the
  measured internal resistance. This 17C discharge rate is within the
  specified rating for this high-power cell, of 32C continuous discharge
  and 55C for 10-s peaks. The characterization tests were done on bare
  cells in a background room temperature of 25 C.
  The resistance difference between the most and least resistive
  cells was 24.7%. The maximum difference in capacity in this same
  batch of cells (one full discharge cycle at 17C continuous discharge
  current) was 3.6%. For the purposes of this experiment, the differences
  in initial capacity were considered to be negligible compared
  to the differences in internal resistance.
  
  [3] H.-G. Schweiger, O. Obeidi, O. Komesker, A. Raschke, M. Schiemann, C. Zehner,
  M. Gehnen, M. Keller, P. Birke, Sensors 10 (6) (2010) 5604e5625, http://dx.doi.org/10.3390/s100605604