Clarified distinction between running tests in source code vs. installed version

closes #795
adjusted wording on astropy.test() to "may not work"
Add more system/dependency information to py.test output
Add Table.join method
New files for table join
Change to more general way of creating unique column names in join
Initial implementation of metadata merging
Use a mapping from new col names back to originals in join
Add code to merge column and table meta data
Apply the mask for input masked arrays
Fix problem for zero-length input tables
Rename cyjoin.pyx to _np_utils.pyx
Fix downloads URL and add a Troubleshooting section
Change numpy_lt_1pt constant to all CAPS
Add new test file for table utilities such as join
Check for duplicate output column names when merging
Refactor merge_col_meta and merge_table_meta out of Table class
Test column renaming in table join
Xfail tests for numpy 1.4 if masks required
Add tests of column meta merging