Update missing CHANGELOG entries

Hopefully I did not miss much.
Merge pull request #563 from pjg/patch-1

Update missing CHANGELOG entries
Update README.md
Merge pull request #574 from AdamFreemer/patch-1

Update README.md [ci skip]
Adding configurable custom parsers for tags.
Merge pull request #579 from ProGM/implement-custom-filters

Adding additional configurable filters to tags
added popular method

Feature added to find popular tags

Refractored using new lambda syntax, added 'least' method

Removing white space

Fixing test so that taggable counts are set in rails 3.2

Restoring space for readability

Included information on new features 'most_used' & 'least_used'

Editted to display properly
Merge pull request #577 from wegotcoders/popular-feature

Popular feature
Update for rails edge (4.2)
Merge pull request #583 from twalpole/edge

Update for rails edge (4.2)
[Misc]
version bump
added gemfiles for rails 4.2.0.beta1
fixed typos in README
fetch ids without instantiating records when pluck is available