merged branch fabpot/filesystem-optimizations (PR #1155)

This PR was merged into the master branch.

Discussion
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optimized the filesystem loader

* added a cache for templates that do not exist
* sped up Filesystem::exists() by avoiding the creation of exceptions

closes #1096

Commits
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d2b97b4 optimized the filesystem loader

diff --cc CHANGELOG
index 664765b,d4b5f5f..9276586
--- 1/CHANGELOG
--- 2/CHANGELOG
+++ b/CHANGELOG
@@@ -1,6 -1,6 +1,7 @@@
  * 1.14.0 (2013-XX-XX)
  
+  * improved the performance of the filesystem loader
 + * fixed a crash of the C extension on an edge case
   * fixed the filesystem loader cache when a template name exists in several namespaces
   * added support for calling macros defined in a template without importing them first
   * added support for named arguments for macros