API: user api for new sql functionality
Starting a new issue to discuss this. See merged PR #5950 and follow-up issue #6292.

Summary of situation:

**Current (0.13)**:
- top-level `read_sql`, `read_frame`, `write_frame` and `DataFrame.to_sql`
- in sql module: `uquery`, `tquery`, `has_table` (+ `read_sql`)

**New after #5950**
- top-level `read_sql` and `DataFrame.to_sql` (but not yet used in the docs)
- in sql module: `read_sql`, `read_table`, `to_sql`, `has_table`, `execute`
- `read_frame`, `write_frame`, `uquery`, `tquery` are deprecated

**Points to dicsuss**
- docs: use top-level functions `read_sql` and `DataFrame.to_sql` instead of functions from `sql`
- `sql.read_table` vs existing `pd.read_table`. Is this a problem?
- merge functionality of `read_table` and `read_sql` or not (see this comment of @mangecoeur to not do that: https://github.com/pydata/pandas/pull/5950#issuecomment-33174797)
