use dpi units
Bring back PPI-based units
Add inches to line height
Merge pull request #114 from opengenius/master

Restores PPI units, needs to be combined with PR #129.  Also needs fixes to work correctly as the current PPI is hard coded to 100 ppi which applies to pretty much nothing.  See issue #138.  Will be augmented int he future with density independent pixel units (DP) as defined in issue #139
Merge pull request #129 from SiPlus/patch-1

Restores PPI units, combined with PR #114.  Also needs fixes to work correctly as the current PPI is hard coded to 100 ppi which applies to pretty much nothing.  See issue #138.  Will be augmented int he future with density independent pixel units (DP) as defined in issue #139
Fixed duplicate code due to merging two related pull requests.
Merge remote-tracking branch 'librocket/master'
Fixes issue #113, DataGrid Crash On Overflow
 Suggested fix by LastSupertim