fix cache implementation

the cache would behave badly when the same item was insert()ed twice:
it would add the size twice to memory consumption, but would never substract
it twice when purging the (actually single) object from cache. So the cache
would seem to fill up while in fact it wasn't.
update base

Blitbuffer RGB16 fixes
(should only apply to screenshots for now)
Merge pull request #1143 from hwhw/master

Fix cache implementation, pull blitbuffer fix
fix memory calculation for cache insert decision, simplify drawing/rendering
pull latest base

contains fix/workaround for freeing memory used by MuPDF
Merge pull request #1144 from hwhw/master

Memory related fixes
Merge pull request #1145 from NiLuJe/master

Some more refresh fixes...
fix #1142 by adding 'show page overlap' option for EPUB doc
And 'show page overlap' option is disabled in "page" mode for EPUBs,
and in fit to page/*height zoom mode for PDFs.
fix #1149 by adding 'Network setting' in filemanager
screensaver options only apply to Kobo devices
dismiss 'Opening file' dialog once reader view is ready
fix #1154
And koreader release version is normalized to a 10 digits containing
a four digits year field followed by a two digits month field followed
by a four digits revision field like "2014041079".
Merge pull request #1152 from chrox/fix_1142

fix #1142 by adding 'show page overlap' option for EPUB doc
Merge pull request #1153 from chrox/fix_1149

fix #1149 by adding 'Network setting' in filemanager
Merge pull request #1159 from chrox/fix_1155

dismiss 'Opening file' dialog once reader view is ready
Merge pull request #1160 from chrox/fix_1154

fix #1154
File manager improvements: added delete confirmation dialog and success/failure message

This makes deleting files from within KOReader less scary and also provides the initial framework for handling different exit codes for #941.
Merge pull request #1162 from Frenzie/master

File manager improvements: added delete confirmation dialog and success/failure message
"File to remove" DEBUG message was empty

You can't get the full file path of a file that no longer exists, obviously.
Merge pull request #1163 from Frenzie/master

"File to remove" DEBUG message was empty