CB-5006 Fix searchpath not being passed to dependent plugins.
CB-5802 Updated version and RELEASENOTES.md for release 0.18.0
Bump node-xcode dependency version
Update node-xcode dependency to 0.6.6
CB-5299 Cache pbxproj to avoid re-parsing it for each plugin.

Parsing pbxproj is very slow (~250ms on my machine). It is done for each plugin
that needs to add frameworks to the ios project. This change is a quick and
dirty fix to allow caching the parsed pbxproj object so that it can be reused for
multiple plugins during prepare.

Timings on a test project for cordova prepare ios:
before: 1.623s
after:  0.981s
CB-5804: added repo & issues to plugman publish
CB-5891 Fix engine check when path has spaces.
updated release notes
Setting version to 0.19.0
Allow simultaneous install/uninstall of multiple plugins
Convert tabs to spaces
Cache and reuse already parsed ios project files.
Fixed exception propagation