[AbstractExternalSource] Quickly validate specs

[Validator] Allow specifying only the subspec name

[xcconfig_helper] Remove the library name's extension when adding it in the "linker flags" build setting to support dynamic libraries.

Set the CFBundleIdentifier using the PRODUCT_BUNDLE_IDENTIFIER build setting

This is how Xcode 7 likes to do things

Changes search index format to fasten `pod search --full` command.

    - Override update_git_repo method of Source class in SourcesManager, and uses Executable instead of backticks. This provides streaming command output to STDOUT.

- Previous implementation was having the drawback of traversing and performing `gsub` on all index strings for each pod specification.
- New implementation stores words as keys and list of pods containing corresponding word inside their specification as the values for corresponding hash keys. Therefore, while searching for a query, we only need to check if query is matched with any key in index hash, if so, we will add corresponding list of spec names to a Set object. Resulted Set object gives us the search result. Using this policy, sources manager can perform a faster search operation.

[PodspecFinder] Only consider toplevel podspecs

Improve message when there is no spec in repos for a given dependency