add a jnius.detach() method to detach a thread that used some jni.

This cannot be automatic as jnius do not control the thread creation, so it cannot add a callabck to autocall this method.
It must be added by the user before the thread leave, if it used pyjnius inside.

Ref #107
Ref #97

jnius/jni.pxi

@@ -401,4 +401,5 @@ cdef extern from "jni.h":
 
     ctypedef struct JNIInvokeInterface:
         jint        (*AttachCurrentThread)(JavaVM *, JNIEnv **, void *)
+        jint        (*DetachCurrentThread)(JavaVM *)
 

jnius/jnius.pyx

@@ -87,7 +87,7 @@ Python::
 
 __all__ = ('JavaObject', 'JavaClass', 'JavaMethod', 'JavaField',
            'MetaJavaClass', 'JavaException', 'cast', 'find_javaclass',
-           'PythonJavaClass', 'java_method')
+           'PythonJavaClass', 'java_method', 'detach')
 
 from libc.stdlib cimport malloc, free
 from functools import partial

jnius/jnius_env.pxi

@@ -19,3 +19,7 @@ cdef JNIEnv *get_jnienv() except NULL:
     jvm[0].AttachCurrentThread(jvm, &env, NULL)
     return env
 
+
+def detach():
+    jvm[0].DetachCurrentThread(jvm)
+