LaTeX writer: Changed figure defaults to htbp.

This prevents "too many unprocessed floats."  Resolves
Issue #285.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -163,7 +163,7 @@ blockToLaTeX (Plain lst) = inlineListToLaTeX lst
 blockToLaTeX (Para [Image txt (src,tit)]) = do
   capt <- inlineListToLaTeX txt
   img <- inlineToLaTeX (Image txt (src,tit))
-  return $ "\\begin{figure}[htb]" $$ "\\centering" $$ img $$
+  return $ "\\begin{figure}[htbp]" $$ "\\centering" $$ img $$
            ("\\caption{" <> capt <> char '}') $$ "\\end{figure}" $$ blankline
 blockToLaTeX (Para lst) = do
   result <- inlineListToLaTeX lst

tests/writer.latex

@@ -788,7 +788,7 @@ or here: <http://example.com/>
 
 From ``Voyage dans la Lune'' by Georges Melies (1902):
 
-\begin{figure}[htb]
+\begin{figure}[htbp]
 \centering
 \includegraphics{lalune.jpg}
 \caption{lalune}