Reformat table/operations.rst and add note about row ordering
Clean up join doc strings and formatting
Rework get_merge_descrs to take list of arrays

Previously get_merge_descrs() took an explicit left and right array.
It was updated, along with common_dtype(), to accept a list of arrays.
This is in preparation for other table operations like horizontal
and vertical stacking of lists of tables.
Change import _np_utils to explicit relative import
Use simple count function instead of collections.Counter (not in 2.6)
Test for masked key elements and add various exception tests
Merge pull request #861 from mdboom/unit/lightyear

units: u.lightyear and u.yr
Merge pull request #892 from mdboom/unit/simplify-arcsecond

Units: Improve arcsecond definitions
Merge pull request #865 from mdboom/console/hide-download-progress

Suppress "Downloading" message in download_file()
Remove mention of PYTHONPATH tinkering
Fix #908, broken by #865.  Makes show_progress == False work correctly
Fix docbuild broken with #892.  The latex formatting of arcsec used a non-standard latex symbol.  Also fixes a bug where the latex symbol of a named unit wasn't always being selected over the Python default.
Added support for constructing a table from a list of dicts
Issue warning when merging conflicting column attributes
Update doc example to use Table.read not io.ascii.read
Fix reference-counting bugs caught by cpychecker.
Merge pull request #911 from mdboom/unit/milliarcsecond-doc-build

Fix docbuild broken with #892.
Further simplification
Add the background graphic for the Windows installer.
Update the documentation to accurately reflect the reality