ENH: add toplevel eval function

This also adds a new directory in the top level pandas directory, called
"computation". Further development on out-of-core processing should be placed
in this directory.
ENH: add string testing capability to core.common
ENH: add randbool testing function to create random boolean arrays
ENH: beef up the alignment algo
CLN/ENH: use values because we cannot depend on the array interface
ENH: clean up indexing and deal with series axis stuff
TST/ENH: add wacky alignment tests
TST/CLN: get rid of generated tests
TST: add tests for pandas objects types
TST: add datetime index type in tests
TST: add conditionals for python testing
CLN: use format rather than str which does not work for unicode objs in python2
BUG: add more clear type testing in datetimeindex join method
TST: check for compatible period indexed axes and fail otherwise