Merge pull request #156 from thotypous/master

Update libpng
build libpng with libm
build leptonica with libpng for PNG I/O
make base test and front test
Merge pull request #157 from chrox/master

make base spec and front spec separately
switch to www.example.com for HTTP request test
update evernote-sdk
add method to export src bitmap to PNG
update evernote sdk
add dummy mode of framebuffer in emulator for testing
Merge pull request #158 from chrox/master

update evernote sdk to support image resources
fix(build): support ccache for asm compiler
feat(build): add fetch.sh script for gettext lib
feat(build): fetch & build gnu gettext
feat(gettext): add ffi luajit binding for gettext
add(test): unit test for gettext ffi binding
update gitignore
fix(build): clean gettext & set libpng source dir
refactor(koreader-base): use gettext ffi binding
minor(build): use a faster gnu mirror for gettext download