Cleanup indexing api, Where/Select with indices, minor tweaks
Added zipInto and zipAlignInto for Series and Frame with tests, plus one bug fix same as issue #30 but for series

Conflicts:
	src/SeriesModule.fs
	tests/FSharp.DataFrame.Tests/Series.fs
Test 'Can join series with lookup and skip over missing values' currently fails

Conflicts:
	tests/FSharp.DataFrame.Tests/Series.fs
Better handling of incomparable index keys, add zip
Merge changes, reimplement zipping
Fix #41 and minor edits