Add InProgress.import to load transaction data from a file. r=emily

It's not streaming, but it'll do.

src/conn.rs

@@ -10,6 +10,14 @@
 
 #![allow(dead_code)]
 
+use std::fs::{
+    File,
+};
+
+use std::io::{
+    Read,
+};
+
 use std::path::{
     Path,
 };
@@ -385,6 +393,14 @@ impl<'a, 'c> InProgress<'a, 'c> {
         self.transact_entities(entities)
     }
 
+    pub fn import<P>(&mut self, path: P) -> Result<TxReport>
+    where P: AsRef<Path> {
+        let mut file = File::open(path)?;
+        let mut text: String = String::new();
+        file.read_to_string(&mut text)?;
+        self.transact(text.as_str())
+    }
+
     pub fn rollback(self) -> Result<()> {
         self.transaction.rollback().map_err(|e| e.into())
     }

src/errors.rs

@@ -40,6 +40,7 @@ error_chain! {
         EdnParseError(edn::ParseError);
         Rusqlite(rusqlite::Error);
         UuidParseError(uuid::ParseError);
+        IoError(::std::io::Error);
     }
 
     links {