Implemented multi member gz decoder

Added a MultiDecoderReader and MultiDecoderReaderBuf to gz. I moved
the read header code to a separate function to avoid code duplication.

In order to keep the reader but reset the decoder a reset_data method
has been added to deflate.rs and a reset method to the CrcReader.

Tests for multimember gz files are in the test directory.

src/crc.rs

@@ -35,6 +35,11 @@ impl Crc {
             ffi::mz_crc32(self.crc, data.as_ptr(), data.len() as libc::size_t)
         };
     }
+
+    pub fn reset(&mut self) {
+        self.crc = 0;
+        self.amt = 0;
+    }
 }
 
 impl<R: Read> CrcReader<R> {
@@ -56,6 +61,10 @@ impl<R: Read> CrcReader<R> {
     pub fn inner(&mut self) -> &mut R {
         &mut self.inner
     }
+
+    pub fn reset(&mut self) {
+        self.crc.reset();
+    }
 }
 
 impl<R: Read> Read for CrcReader<R> {

src/deflate.rs

@@ -289,6 +289,14 @@ impl<R: BufRead> DecoderReaderBuf<R> {
         mem::replace(&mut self.obj, r)
     }
 
+    /// Resets the state of this decoder's data
+    ///
+    /// This will reset the internal state of this decoder. It will continue
+    /// reading from the same stream.
+    pub fn reset_data(&mut self) {
+        self.data = Decompress::new(false);
+    }
+
     /// Acquires a reference to the underlying stream
     pub fn get_ref(&self) -> &R {
         &self.obj