Update nested-set.markdown

Change doubled 'tree_left' to 'tree_right'.
Merge pull request #282 from thetreew/patch-3

Update nested-set.markdown
Update query-cache.markdown

Fix 'custom cache' example code.
Merge pull request #283 from thetreew/patch-4

Update query-cache.markdown
Update validate.markdown (text formatting)
Update validate.markdown
Merge pull request #284 from thetreew/patch-5

Update validate.markdown (text formatting)
Migration database needed for tests running
Merge pull request #285 from nekufa/patch-1

Migration database needed for tests running
Fix Propel's version for Composer

The greater or equal operator is only available for stable releases.
For now, we should rely on an explicit version ; we will update
accordingly when a stable release will be out.

Refs propelorm/Propel2#601
Some corrections regarding propelorm/Propel2/issues/480
typo
Fixed native type for binary columns and changed object sql to binary.
added release 1-7-1
Merge pull request #287 from stevleibelt/master

added propel1 release 1-7-1 to blog
propel2 alpha3 release note
fixed typo
Update 2014-04-16-propel2-alpha3-released
added composer install instructions
Added compatibility index page