Replace 'PX::' with 'self::'
Tweak the Regular Expressions

- put the regex that finds tag attribute names into a static
  variable ($valid_attr)

- update the attribute name regex to support numbers as well as
  underscores(_)

- update 'const' regexs to only validate on uppercase names

- update 'var' regexs to only validate on valid php variable names
  (ie. they can't start with a number)
Allow the Cache Folder to be set via PX::set_cache_dir()
Change the name of the cached manifest file

- change the file extention to '.json' as that is what the file
  contains

- replace the '.' at the beginning of the filename since there is
  no reason the file needs to be hidden
Update regexs in PX::get_manifest() to prevent false positives

- make the equal sign(=) required in the regexs to prevent 'const'
  being triggered on $innerHTML (NOTE: this will prevent a variable
  being set in the 'defaults' array for parameters that don't have
  a default value set)
Remove Unused/Redundant Code
Save the Manifest in a Private Static Variable
Turn PX::template() into a generic file parser (PX::run_file())
Limit Errors/Warnings when running without a Cache Directory

Check if the cache directory is writable before creating a manifest
file, and throw a PHP Notice if it's not.  This will allow the script
to run without a cache directory since Notices are easily ignored.
Delete old manifest files
Update cache directory get/set functions

- don't check if directory is writable when setting the cache
  directory variable

- create public function to retrieve the current cache directory
Update README, add 'cache' section / formatting
Merge pull request #1 from triple-j/fixes

Fixes