tuttleHost: don't try to reload an unsupported plugin

* We always try to reload a plugin if the binary can't be loaded
(something wrong in the environment, like LD_LIBRARY_PATH). But if the
binary is loaded and a plugin use an unsupported OpenFX feature, we can
store the binary in the plugin cache (without unsupported plugins).
So we don't try to reload unsupported plugins anymore.

* Don't save "_knownBinFiles" in plugins cache. The information is
already in "_binaries"

Issue #292
tuttleHost: one ofx binary could contain supported AND unsupported plugins

If there is an error while loading a plugin, we should load all other
plugins from the same binary.

Issue #292
tuttleHost: keep information about unsupported plugins

Add the information that a plugin is not supported to "sam do -n".
This information is saved into the plugins cache.

Issue #292
tuttleHost, sam tools: setLogLevel from int or string

Init value of tuttleHost log level could be modified by an environment
variable "TUTTLE_LOG_LEVEL".

Issue #293
scripttest: syntax bug fix
InputBufferWrapper: fix for python3 binding

img.tostring() on a Numpy array returns a "bytes" object in python 3
(and no more "str", because "str" is now a unicode string in python 3).

Issue #294
tuttleHost pyTest: fix test

The buffer orientation is top to bottom... but we need to reinterpret
the pointer to declare it as a bottom to top.
This commit is a simple fix to make tests working.