ENH: eval function
Provide a top-level `eval` function, something like:

`pd.eval(function_or_string,method=None, **kwargs)`

to support things like:

1) out-of-core computation (locally) (see #3202)

2) string evaluation which does not use python lazy evaluation
 (so pandas can process effiiently)

`pd.eval('df + df2',method='numexpr')` (or maybe default to numexpr)

see also:
http://stackoverflow.com/questions/16527491/python-perform-operation-in-string

3) possible out-of-pandas evaluation
`pd.eval('df + df2',method='picloud')`
http://www.picloud.com/ (though they seem to have really old versions of pandas),
but I think they handle it anyhow
